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Integrative square-grid triboelectric nanogenerator as avibrational energy harvester and impulsive force sensor
Chuan He1,2, Weijun Zhu3,4, Guang Qin Gu1,2,5, Tao Jiang1,2, Liang Xu1,2, Bao Dong Chen1,2, Chang Bao Han1,2,
Dichen Li3,4, and Zhong Lin Wang1,2,6 ()
1 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China 2 CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China 3 State Key Laboratory of Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi’an 710049, China 4 Collaborative Innovation Center of High-End Manufacturing Equipment, Xi'an Jiaotong University, Xi’an 710049, China 5 University of Chinese Academy of Sciences, Beijing 100049, China 6 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
(TENGs) [6–14], have been developed as power sources
for wearable electronics and sensor networks. Among
them, TENG is of great interest for capturing low-
frequency mechanical energy owing to its low-cost
fabrication and excellent robustness [15, 16]. Based
on the coupling of the triboelectric effect and elec-
trostatic induction, different TENG structures have
been designed for harvesting mechanical energy of
different forms, e.g., human motion [9, 10, 12], vibration
[6–8], and water waves [11]. Among these, vibration
is one of the most common mechanical motions
ubiquitously available in our living environment.
Spring-assisted TENGs were previously introduced
to harvest vibrational energy, where the springs were
used to move the triboelectric layers apart when the
TENGs were subjected to external vibrations [6–8].
However, there are certain limitations to spring-
assisted structures. First, the use of springs typically
results in a relatively bulky volume of the TENGs.
Secondly, an additional space is required for the
vibrational part. Thirdly, considerable vibrational energy
loss might occur when the springs become loose.
To avoid these limitations, Wen et al. [11] reported
a wavy TENG structure that allows self-restoration
without using extra springs. In addition, package
structures of TENGs based on the internal vibration of
the oscillators have also been reported. The oscillators
that have been used thus far are polymer-coated metal
[17], polytetrafluoroethylene (PTFE) powders [18], or
ferrofluid [19].
In this article, we demonstrate a square-grid TENG
(SG-TENG) for harvesting vibrational energy and
sensing impulsive forces. The demonstrated SG-TENG
employs a package structure and uses a 3D-printed
square grid as the frame. Each square of the grid is
filled with an aluminum (Al) ball as the oscillator.
The grid structure allows the SG-TENG to operate at
different vibrational angles. Owing to the special design
and small size, the SG-TENG can easily be scaled
and integrated into other structures. By connecting
two SG-TENGs in parallel, both open-circuit voltage
(VOC) and short-circuit current (ISC) are greatly increased
over the entire vibrational frequency range. Furthermore,
when integrated with a table tennis racket, the
SG-TENG can harvest the vibrational energy of the
racket from hitting ping pong balls during game play.
Moreover, the SG-TENG integrated into a focus mitt
can not only count the total number of punches
but also track the force applied in every impact. The
acquired data allow athletes to monitor their status
and improve their performance during training.
2 Results and discussion
The structural design of the SG-TENG is schematically
illustrated in Figs. 1(a) and 1(b). As shown in Fig. 1(a),
the SG-TENG has a sandwich structure consisting
of four parts. In the middle, a 30 × 30 square grid
frame (83 mm × 83 mm × 2 mm) is fabricated using
stereolithography (SL) and filled with Al balls. The Al
balls act as both an oscillator and electropositive
triboelectric layer. On each side of the square grid is a
layer of PTFE film placed on top of an Al plate. Here,
the PTFE film was chosen as the electronegative layer
owing to its ability to attract electrons, whereas the
Al plate acts as both an electrode and a protective
layer. It can be seen in Fig. 1(b) that each square of
the grid contains a single Al ball and the side length
of the square and the diameter of the Al ball are 2
and 1 mm, respectively. This structure allows the
vibration of the Al balls inside the SG-TENG when
an external vibration is applied. Figures 1(c) and 1(d)
show photographs of the side view of the SG-TENG
and the front view of the square grid, respectively.
The detailed fabrication process of the SG-TENG is
presented in the Experimental section.
Figure 2 illustrates the working principle of the
SG-TENG. Considering that the SG-TENG has a grid
structure, we only demonstrate one square unit for
Figure 1 Structural design of the SG-TENG. (a) Schematic illustration of the device structure. (b) The square grid and Al balls inside. (c) Photograph of the side view of the SG-TENG. (d) Photograph of the front view of the 3D-printed square grid.
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3 Nano Res.
Figure 2 The working principle of the SG-TENG.
clarification. According to the triboelectric series,
when the Al ball comes into contact with the PTFE
films, the electrons will inject from the Al ball to the
surfaces of the PTFE films through triboelectrification
[11]. Thus, the total amount of positive triboelectric
charges on the Al ball should be the same as the
negative triboelectric charges on the PTFE films.
Initially, as shown in Fig. 2(i), the Al ball is in contact
with the PTFE film at the bottom, and thus the
negative charges are attracted to the bottom electrode,
leaving the same amount of positive charge on the
top Al electrode. The Al ball then moves upward
when the SG-TENG is subjected to an external
vibration. As the Al ball approaches the top, the
top electrode has a higher potential than the bottom
electrode, and hence the electrons transfer from the
bottom to the top in the external circuit (see Fig. 2(ii)).
Once the Al ball reaches the top PTFE film, all electrons
are transferred to the top electrode, as shown in
Fig. 2(iii). When the Al ball takes the reverse course, a
reverse transfer of the electrons occurs through
the external circuit (see Fig. 2(iv)). Finally, the Al ball
returns to the bottom of the PTEF film, and a full
cycle is completed (see Fig. 2(i)).
To evaluate the performance of the SG-TENG, an
electrodynamic shaker (Labworks, Inc.) was used to
produce sinusoidal vibrations with a fixed amplitude
and tunable frequency. After attaching the SG-TENG
to the shaker, the output dependence of the SG-TENG
on the vibration frequency was measured at vibrational
angles of 0°, 45°, and 90°. The vibrational angle, α,
is defined as the angle between the surface of the
SG-TENG and normal to the ground. It should be
noted that the surface of the SG-TENG is always
perpendicular to the vibrational direction. As previously
described, the sinusoidal vibration applied to the
SG-TENG induces the vibration of the Al balls inside,
and hence an alternating current (AC) is produced in
the external circuit. Figures 3(a)–3(c) show the electrical
output of the SG-TENG at α = 0°, 45°, and 90°,
respectively. The VOC and ISC of the SG-TENG over a
vibration frequency at α = 0°, 45°, and 90° are plotted
in Figs. 3(a)(i), 3(b)(i), and 3(c)(i), respectively. The
vibrational frequency ranges from 10 to 180 Hz,
which covers most of the ambient vibrations in our
daily life [20]. At α = 0°, 45°, and 90°, the bandwidths
for the voltage are 92.6, 89.3, and 88.3 Hz, respectively,
and for the current are 61.94, 84.6, and 103.3 Hz,
respectively; in addition, the measured peak-to-peak
values of VOC are 3.76, 5.66, 5.28 V, respectively, whereas
the amplitudes of ISC are 0.37, 0.41, and 0.39 μA,
respectively. We can see that as α increases from 0°
to 90°, the electrical output also increases over the
vibrational frequency, particularly within the low
frequency range, which leads to an increase in the
bandwidth. The reason for this can be ascribed to the
fact that, at an angle of 90°, the Al balls are at rest at
the bottom of the SG-TENG, and thus more vibrational
energy can be transferred to them than to the Al balls
at an angle of 0°, which are at rest at the side of the
SG-TENG.
Moreover, VOC and ISC of the SG-TENG in the time
domain at α = 0°, 45°, and 90° are also depicted in
Figs. 3(a)(ii) and 3(a)(iii), 3(b)(ii) and 3(b)(iii), and 3(c)(ii)
and 3(c)(iii), respectively. The electrical signals at 20,
40, 60, and 80 Hz are chosen here for comparison.
The insets in Figs. 3(a)(iii), 3(b)(iii), and 3(c)(iii) depict
the corresponding transferred charges between two
electrodes. As the frequency increases, the signal
evolves from a pulsed output to an oscillatory output;
in addition, the peak current also increases, whereas
the total charge transferred during one cycle remains
almost constant. At a frequency of 80 Hz, the total
charge transferred at α = 0°, 45°, and 90° is 1.65, 1.68,
and 1.35 nC, respectively. These results indicate that
the SG-TENG is capable of harvesting vibrational
energy over a broad bandwidth and at different
vibrational angles.
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4 Nano Res.
One of the features of the SG-TENG is its scalability,
and by connecting the SG-TENGs in parallel, the total
output can be increased with an increased number of
SG-TENGs. To prove this, we measured the frequency
response of two parallel-connected SG-TENGs. A
comparison of VOC and ISC for a single SG-TENG and
two parallel-connected SG-TENGs is illustrated in
Figs. 4(a)(i) and 4(a)(ii), respectively. The measurements
were performed at a vibrational angle of 90°. Clearly,
VOC and ISC of the two parallel-connected SG-TENGs
are greatly increased over the full vibrational frequency
range. Compared to the single SG-TENG, the parallel-
connection increases the contact area between the
triboelectric layers, which produces more triboelectric
charges, thus leading to an enhancement in the
electrical output. At a frequency of 80 Hz, the peak-
to-peak value of VOC and the amplitude of ISC are 2.4
and 3.9 times greater than those of a single SG-TENG,
respectively. Therefore, a higher electrical output is
expected as the number of SG-TENGs increases.
As mentioned before, owing to its small size, the
SG-TENG can be easily integrated into any vibrational
surfaces or structures for harvesting the vibrational
energy. For example, as shown in Fig. 4(b), we
demonstrated the ability of the SG-TENG for harvesting
the vibrational energy of a table tennis racket. Because
the SG-TENG is only 4 mm thick (see Fig. 1(c)), it
can be integrated into the racket; in this study, we
directly attached a SG-TENG to a racket for simplicity.
Figure 4(b)(i) shows an image of a ping pong ball
bouncing on the racket, with an image of the SG-TENG
being attached to the racket shown in the inset of
Fig. 4(b). The amounts of VOC and ISC generated by
the SG-TENG are shown in Figs. 4(b)(ii) and 4(b)(iii),
respectively, and enlarged views of the highlighted
VOC and ISC are shown in the corresponding insets. As
the ball bounces against the racket, a series of electrical
signals is generated each time the ball hits it (Video S1
in the Electronic Supplementary Material (ESM)). The
average output voltage of the SG-TENG is about 4.6 ±
Figure 3 The VOC and ISC of the SG-TENG at different vibrational angles α. (a) α = 0°, (b) α = 45°, and (c) α = 90°. The insets in (a)(iii), (b)(iii), and (c)(iii) show the corresponding transferred charges between two electrodes.
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5 Nano Res.
0.9 V, whereas the average output current is 0.19 ±
0.02 μA. Furthermore, the electrical performance of
the integrated SG-TENG during game play was also
evaluated by hitting a series of ping pong balls using
the racket (see Fig. 4(b)(iv)). The values of VOC and ISC
of the SG-TENG during consecutive strokes are shown
in Figs. 4(b)(v) and 4(b)(vi), respectively, where the
signals generated in a single stroke are highlighted. A
single stroke consists of two sequential processes: the
swinging of the racket and a direct hit on the ball by
the racket. As indicated in Figs. 4(b)(v) and 4(b)(vi),
the entire process is delicately disclosed in the obtained
signal. Because the swing process precedes the hitting
process in a single stroke, the electrical signal produced
is composed of two parts: signals generated by the
swing process and signals generated by the hitting
process that follows. During game play, the swing of
the SG-TENG produces an average output voltage
of 9 ± 1 V and an average output current of 0.05 ±
0.02 μA, whereas a direct hit by the racket generates
an average output voltage of 10.9 ± 0.6 V and an
average output current of 0.09 ± 0.02 μA (Video S2 in
the ESM). This demonstration proves that an integrated
SG-TENG can effectively harvest the vibrational energy.
In addition, the SG-TENG is sensitive to different
types of vibrations, i.e., swinging and hitting during
play, and thus the SG-TENG also has a great potential
in sensor applications.
In addition to harvesting vibrational energy, the
capability of the SG-TENG as an impulsive force sensor
is demonstrated. Figure 5(a)(i) shows the dependence
of ISC on the impulsive force applied on the SG-TENG.
It is clear that the amplitude of ISC increases as the
impulsive force increases. The relationship between
Figure 4 (a) Comparison of (i) the VOC and (ii) ISC for a single SG-TENG and two parallel-connected SG-TENGs. (b) Demonstration of the SG-TENG as a vibrational energy harvester. (i) Photograph of a table tennis racket with an integrated SG-TENG. As the ball bounces on the racket, VOC and ISC generated by the SG-TENG are as shown in (ii) and (iii), respectively. The insets are the respectively enlarged views of one cycle of VOC and ISC. (iv) The electrical performance of the integrated SG-TENG during game play evaluated when hitting a series of ping pong balls using the racket. The corresponding VOC and ISC of the SG-TENG are shown in (v) and (vi), respectively.
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6 Nano Res.
the amplitude of ISC and the impulsive force is plotted
in Fig. 5(a)(ii). From Fig. 5(a)(ii), we can see that the
amplitude of ISC is approximately linearly proportional
to the impulsive force. This linearity is crucial for the
SG-TENG as an impulsive force sensor. As is well
known, the impulsive force is commonly encountered
in various combat sports, such as boxing, taekwondo,
and kickboxing. Taking boxing as an example, a focus
mitt is a padded target that is generally used for
training boxers and other combat athletes. For this
study, we integrated a SG-TENG into the focus mitt,
as shown in Fig. 5(b). When the focus mitt is punched
repeatedly, a series of pulse signals is generated
(Video S3 in the ESM). The electrical signals of the
integrated SG-TENG punched by persons A and B
are illustrated, where VOC and ISC generated by person
A are shown in Figs. 5(b)(i) and 5(b)(ii), respectively,
and VOC and ISC generated by person B are shown in
Figs. 5(b)(iii) and 5(b)(iv). The insets in Figs. 5(b)(i)
through 5(b)(iv) show enlarged views of the corres-
ponding highlighted pulse signals. Because each
punch produces a pulse signal, the SG-TENG can be
utilized to count the total number of punches during
training. In the meantime, as indicated in Fig. 5(a),
the SG-TENG is also able to track the force of every
punch. The frequency and magnitude of the punches
collected by the SG-TENG can help athletes monitor
their status and improve their performance during
training.
3 Conclusions
In conclusion, we demonstrated a square-grid TENG
as a vibrational energy harvester and an impulsive
force sensor. The design of the SG-TENG allows it to
harvest the vibrational energy over a broad bandwidth
at different vibrational angles (α = 0°, 45°, and 90°).
The SG-TENG can be easily scaled, and by connecting
Figure 5 Demonstration of the SG-TENG as an impulsive force sensor. (a) (i) The ISC of the SG-TENG under different impulsive forces. (ii) Amplitude of ISC as a function of impulsive force and the linear fit of the experiment data. (b) Photograph of a focus mitt withan integrated SG-TENG as an impulsive force sensor. The VOC and ISC of the focus mitt punched repeatedly by persons A and B.
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7 Nano Res.
two SG-TENGs in parallel, the values of both VOC
and ISC are greatly increased over the full vibrational
frequency range. At a frequency of 80 Hz, the peak-
to-peak value of VOC and the amplitude of ISC are 2.4
and 3.9 times as large as those of a single SG-TENG.
By integrating the SG-TENG into a table tennis racket,
the SG-TENG can harvest vibrational energy from
hitting the ping pong balls using the racket. A direct
hit by the racket generates an average output voltage
of 10.9 ± 0.6 V and an average output current of 0.09 ±
0.02 μA. Furthermore, the SG-TENG can be used as
the impulsive force sensor. We demonstrated the
ability of applying the SG-TENG to boxing training
to count the total number of punches and track the
force of every punch. Owing to its lightness in weight
and thinness, as well as its capability of being scaled
and integrated, the SG-TENG has significant potential
in energy harvesting and sensing applications.
4 Experimental section
The SG-TENG consists of a square grid frame, Al
balls, and two polytetrafluoroethylene (PTFE) films
on Al plates. The 30 × 30 square grid has a thickness
of 2 mm and a side length of 83 mm. The square grid
is fabricated using epoxy acrylate with a SL, which
has a resolution of ±0.1 mm. The side length of the
square is 2 mm. For each square, there is an Al ball
with a diameter of 1 mm filled inside. On each side of
the square grid is a layer of PTFE film placed on top
of the Al plate. The Al plate has a thickness of 1 mm,
whereas the thickness of the PTFE film is 80 μm. All
of the electrical measurements of the SG-TENG were
applied using a Keithley 6514 System Electrometer. A
function generator (Stanford Research Systems DS345)
and a linear power amplifier (Labworks PA-141) were
used to produce the sinusoidal oscillations. A vibration
shaker (Labworks ET-126B-4) was used to simulate
mechanical vibration. The dynamic force applied was
measured using a force gauge (HP-50) mounted on a
linear motor.
Acknowledgements
Supports from the “thousands talents” program for
the pioneer researcher and his innovation team, the
National Key R&D Project from Minister of Science
and Technology, China (No. 2016YFA0202704), National
Natural Science Foundation of China (Nos. 51432005,
51608039, 5151101243, 51561145021, and 51505457),
China Postdoctoral Science Foundation (No.
2015M581041), and Natural Science Foundation of
Beijing, China (No. 4154090) are appreciated.
Electronic Supplementary Material: Supplementary
material (Videos S1–S3 demonstrate the electrical
performance of the SG-TENG that being integrated
into the table tennis racket and the focus mitt) is
available in the online version of this article at
https://doi.org/10.1007/s12274-017-1824-8.
References
[1] Liu, W. S.; Jie, Q.; Kim, H. S.; Ren, Z. F. Current progress
and future challenges in thermoelectric power generation:
From materials to devices. Acta Mater. 2015, 87, 357–376.
[2] Bell, L. E. Cooling, heating, generating power, and recovering
waste heat with thermoelectric systems. Science 2008, 321,
1457–1461.
[3] Wang, Z. L.; Song, J. H. Piezoelectric nanogenerators based
on zinc oxide nanowire arrays. Science 2006, 312, 242–246.
[4] Hu, Y. F.; Xu, C.; Zhang, Y.; Lin, L.; Snyder, R. L.; Wang,
Z. L. A nanogenerator for energy harvesting from a rotating
tire and its application as a self-powered pressure/speed